Our main emphasis in the past few years has been to bring the bovine brain argininosuccinase to a state of homogeneity and to establish a detailed characterization of the catalytic, chemical, and physical properties of the enzyme. This work is reaching a state of virtual completion and will thus allow return to several other problems still in progress. Studies on the active site of bovine liver argininosuccinase will be continued. The results of covalent modification of the enzyme with a 14C-bromo analogue of fumarate are in agreement with the stoichiometry expected for the number of active sites. This has formed the basis of the procedures planned for isolation of the radioactive peptide fragment representing the region of the active site. It is also planned to undertake identification of the modified amino acid. To facilitate the latter, alkylated derivatives of the several most likely amino acid candidates will be synthesized as models. Some recent improvements in stabilizing the bromo analogue during storage as well as during interaction with the enzyme will, we expect, facilitate alkylation for preparative scale work. The details of a radioassay for argininosuccinase have been developed with 14C-labeled argininosuccinate to replace slow and troublesome colorimetric procedures. The method depends on product separation by ion exchange chromatography. The results are highly reproducible and the sensitivity has been greatly increased. All procedural details have been checked at all stages by independent assay. The method is being applied to crude homogenates of rat liver and other rat tissues and to a variety of cells grown in culture.